The invention relates to a process for the preparation of a benzene-monocarboxylic acid by oxidation of a monoalkyl-benzene compound in the liquid phase with the aid of a gas containing molecular oxygen in the presence of a catalyst composed of a cobalt and a manganese compound, both of which are soluble in the reaction mixture. Such a process is known from Dutch patent application No. 7,311,187.
In the process according to Dutch patent application No. 7,311,187 benzoic acid is prepared by oxidizing toluene in the substantial absence of a lower fatty acid and/or a halogen compound, and in the presence of a cobalt compound and a manganese compound with a weight ratio between cobalt metal and manganese metal of less than 99,8:0,2, which means that the manganese:cobalt atomic ratio must be more than 1:466. This process has as a disadvantage that the selectivity of the reaction to form the desired benzene-monocarboxylic acid is not satisfactory.